The invention relates to an ultrasonic position detecting device on a linear drive, for the detection of the axial position of a movably arranged drive element in the housing of a linear drive, comprising a sensor device arranged fixedly in relation to the housing in the radially peripheral part of the drive element, said sensor device having an acoustic wave guide extending in the longitudinal direction of the housing and made of a material suitable for magnetostriction and adapted to communicate with an evaluating device, and a magnet arrangement having a permanent magnet, arranged within the housing, and so connected with the drive element that its magnetic field may affect the part, which is at a given time radially adjacent, of the acoustic wave guide.
Ultrasonic position detection devices are disclosed in the prior art and their principle of operation is for example described in the U.S. Pat. No. 3,898,555. In the working embodiment of this patent the measuring system comprises a wire arranged coaxially in a tube, the so-called acoustic wave guide, and consists of a magnetostrictive material. The part, whose position is to be detected, is connected with a permanent magnet, which can run along the acoustic wire guide. A short current pulse caused to flow along the wire produces a traveling magnetic field, which is locally modified by the magnetic field of the permanent magnet so that the acoustic wave guide is subject to a mechanical strain. Accordingly a torsion wave is created in the acoustic wave guide, which is propagated at the speed of light and at the end of the acoustic wave guide is converted into electrical pulses. The time between the occurrence of such electrical pulse and the initial current pulse renders possible the determination of positions.
A linear drive fitted with such a position detection device is disclosed in the European patent publication 0 498 918 A1. The linear drive is a fluid power cylinder, whose drive element constituted by the piston bears a permanently magnetic magnet arrangement, which is constituted by two spaced rod magnets or by a horseshoe magnet. The arrangement is such that the mutually spaced pole surfaces or areas are radially aligned and the emerging field lines are directed right toward the acoustic wave guide.
The design of the known magnet arrangement involves considerable complexity. It is only when the magnetic field meeting with the acoustic wave guide complies with certain criteria as regards density and configuration that reliable measurement is possible with negligible errors. Therefore in the case of different conditions of use--for example in the case of employment on fluid power cylinders--permanent magnet parts have to be selected and arranged on a case to case basis.